I need a reliable low battery early warning system.
I'm using Epson RX8900SA RTC and CR2032 battery. I want to measure battery voltage (probably every 24 hour or more).
How can I do this without draining battery too much?
My first idea is to use low cost J-FET input amplifier as buffer (voltage follower circuit) and feed microcontroller ADC from its output. If this is good idea - which op-amp can I use? TL082 has too high supply requirements for me (I have 3.3V and 5V supplies avaliable, and TL082 needs at least 6V to do anything). Or maybe I can just use regular low cost single supply op-amp like LM358?
I also want to use some mosfet to connect resistor as load (around 10-20kOhm) at the moment of measurement to make measurements more reliable. Maybe I could just connect 10k-20k resistor directly to ADC and connect that (on high side) by mosfet when I take measurement?
My question is totally diffrent from this.
I'm not asking about ohms law. I asked for solution or some improvement/comment for my idea with high impedance measurement with op-amp or mosfet switch.
Come on guys...